Conventional magnetic recording elements that are used for recording sounds or images are generally opaque to visible light regardless of the nature of the magnetic particles used in such elements. For example, motion picture films often are provided with a magnetic sound track which generally is opaque and does not cover that portion of the film used in the projection of images.
Canadian Patent 686,172 shows that a magnetic recording layer may be transparent to visible light when it contains low concentrations of magnetizable particles. According to this patent, such a layer is coated over a layer containing descriptive material which allows a user to simultaneously hear and see certain subject matter. However, this patent points out that the electromagnetic characteristics, i.e., the magnetic recording and reproducing characteristics, of such a layer are inferior to those of conventional magnetic layers as a result of the very low concentration of magnetizable particles.
U.S. Pat. No. 3,782,947 discloses a photographic product which carries magnetic particles distributed across the image area of the product. The particle distribution and sizes are so designed that the composite granularities of the photographic and magnetic recording media are such that the magnetic distribution is essentially transparent in a photographic sense. The quantity of solvents required, however, is unattractive from both an economic and environmental standpoint. Moreover, it is difficult to prepare magnetic recording layers in such a coating process having a thickness of less than about 5 micrometers.
U.S. Pat. No. 4,279,945 discloses a process of preparing magnetic recording elements containing a transparent recording layer. According to this patent, the magnetic recording and reproducing characteristics obtained are comparable to conventional opaque magnetic layers without the need for matching the granularity of a magnetic medium to that of photographic medium. However, the process requires that the layer containing magnetic particles be treated using one or both of the following process steps, (1) compacting the layer while it is in a malleable state to reduce its thickness (e.g., calendaring), or (2) imbibing into the layer a substantially transparent liquid having a refractive index that is substantially the same as that of the binder. Further, as indicated by the Figure therein, the disclosed magnetic layers are substantially opaque at wavelengths less than about 500 nm and thus are not useful in color films. Further, the disclosed process requires that the magnetic recording layer be calendared while it is in a malleable state and/or that a transparent liquid be imbibed into the magnetic recording layer.
Recent patent literature has disclosed commercially viable technologies for a photographic element having a transparent magnetic recording layer for information and data recording and reading purposes. For example, U.S. Pat. Nos. 4,990,276, 5,254,449, 5,427,900 and 5,432,050 describe transparent magnetic recording layers for use in photographic elements wherein organic solvents are used for the preparation of a dispersion containing the magnetic particles. U.S. Pat. No. 4,990,276 in particular discloses the preparation of transparent magnetic recording layers wherein a concentrated dispersion of magnetic oxide particles, a dialkylester of phthalic acid and a dispersing agent, is first formed, which is then diluted with a binder solution to form a composition suitable for coating or casting. The use of dialkylesters of phthalic acid, such as dibutyl phthalate, has been found to be particularly advantageous in the preparation of relatively concentrated magnetic oxide particle dispersions, which upon dilution allow for the formation of transparent magnetic coating compositions substantially free of unacceptable magnetic particle agglomeration and light scattering. In commercial applications of the disclosed process, coating dispersions for transparent magnetic recording layers use pre-dispersed concentrated magnetic oxide compositions (e.g., typically at least 25 weight percent magnetic oxide particle) which are letdown in successive steps to form the coating solution. The first stage is making “grind” or concentrated oxide dispersions in solvent with a dispersant. The magnetic oxide grind is then typically letdown from about 40 to 55% oxide to about 1 to 3% in a stabilizing, film-forming polymer solution termed an “intermediate letdown” which desirably has good shelf-life and colloid stability. Finally, the coating dispersion is made up from binder polymer solutions, the magnetic intermediate, and any additive solutions or particle dispersions and trim solvent amounts.
In commercial applications, a concentrated magnetic oxide particle dispersion similar to that prepared in Examples 1 or 2 of U.S. Pat. No. 4,990,276 may be diluted with cellulose diacetate/cellulose triacetate solutions to the following composition using a high shear mixer for the dilution procedure.
Coating CompositionIngredientsWeight PercentMethylene chloride67.84Acetone24.224Methyl acetoacetate4.844Cellulose diacetate2.6Cellulose triacetate0.12Dibutyl phthalate0.234GAFAC PE-510 (GAF Corp.)0.006dispersing agentCo-γ-Fe2O3 magnetic0.12oxide particlesFluorad FC-431 (3M Corp)0.012coating aidThis coating composition typically may be cast onto subbed polyethylene naphthalate to a dry thickness of about 1.2 microns of cellulose acetate binders containing uniformly dispersed magnetic particles at a laydown of from about 20 mg/m2 to 100 mg/m2. This composition at 3.1% w/w solids coated at the rate of 49.5 g/m2, for example, provides a dry laydown of 1535 mg/m2 and a magnetic particle laydown of 59.4 mg/m2. At these levels of magnetic oxide, coded information can be written and read from the magnetic layers by use of suitable write/playback heads.
While the use of dialkylesters of phthalic acid such as dibutyl phthalate as grind solvents in the preparation of transparent magnetic coating compositions only requires a minor amount of such materials relative to the final coated material (e.g., only about 0.05% w/w of a photographic film roll), for environmental reasons it may be desirable to reformulate the magnetic coating compositions to be essentially free of aromatic di-ester plasticizers. In order to remove the minor amounts of dibutyl phthalate from the transparent recording layer compositions, it would be desirable to provide a functional, cost effective and environmentally benign replacement solvent.
Various conventional solvents or co-solvents have been suggested for use in preparation of magnetic particle dispersions to form coating dispersions and subsequently, either transparent or non-transparent recording layers, including toluene (methyl benzene) and ketones (e.g., methyl ethyl ketone (MEK), methyl isobutyl ketone (MIBK)). While these volatile solvents are capable of providing similar quality results when used as a grind solvent along with a dispersing agent, they have much lower boiling points, flash points and higher odor levels than dibutyl phthalate. The low flashpoint, flammability and higher odor level mean that while these solvents may be functional, they are not as suitable as desired from the manufacturing robustness, safety and capital cost issues.
U.S. Pat. No. 4,419,257 discloses the use of cellosolve/dibasic ester solvent combinations in the preparation of magnetic layer coating compositions designed to have particular solubility parameters after 99% of the solvent is evaporated from the coating layer binder. While dimethyl succinate, dimethyl glutarate and dimethyl adipate are mentioned as dibasic esters, there is no disclosure or suggestion for the use of such materials as the major solvent in the preparation of a magnetic particle grind dispersion in the preparation of a transparent magnetic layer coating composition.
U.S. Pat. Nos. 5,395,743, 5,397,826 and U.S. Pat. No. 5,491,051 disclose the use of methyl acetoacetate in the preparation of a dispersion of magnetic or abrasive particles for use in magnetic recording layers. However, it has been found that methyl acetoacetate appears to react with the iron in the magnetic pigment to result in a reddish colored complex and is therefore not desirable for use as a grind solvent in the preparation of concentrated magnetic particle dispersions for use in transparent magnetic recording layer compositions.